Spot Weld Modeling With Implemented Rupture Criteria - Beta Cae
th5 ANSA & μETA International ConferenceSPOT WELD MODELING WITH IMPLEMENTEDRUPTURE CRITERIAJan Morawski1, Beate Lauterbach1, Ilka Schwarzer1, Laia Ramon-Villalonga1,Michael Tryfonidis2Adam OPEL AG1Beta CAE2KEYWORDS –Spot weld, rupture, heat affected zone (HAZ), automated spot weld realizationABSTRACT –In today’s automotive CAE models the prediction of spot weld rupture is of high importancefor vehicle performance assessments. Coming from single coupon tests, ANSA is utilized toapply all necessary information about the spot welds definition (e.g. weld partners, nuggetdiameter, sheet and nugget material properties and heat affected zones) to the full vehicleCAE model.The advantage of this very detailed modeling is shown exemplarily with a seat console weldperformance within an ECE R14 kind of test setup. Different approaches of modelingtechniques were investigated.This process is implemented efficiently in an automated way by means of ANSA scriptingfunctionality.
th5 ANSA & μETA International Conference1 INTRODUCTIONThe entire body of a modern vehicle typically contains a few thousand spot welds joiningdifferent materials and gage combinations. The consideration of their mechanical propertiesand fracture behavior is a key to accurately predict the response of vehicle structures in thevirtual development process. In case a spot weld detaches, the load which is transferredfrom part to part will change and may result in different deformation results of full vehiclecrashes.In order to be able to reproduce the behavior of all different spot weld joint combinations alarge number of experiments is required. Based on experimental results a comprehensivedatabase for FE analysis containing detailed spot weld information such as nugget and heataffected zone (HAZ) material properties, diameter as well as rupture characteristics needs tobe established.The pre-processor ANSA with its new spot weld scripting functionality is applied to implementall necessary information about the spot weld in the full vehicle FE model in an efficient andautomatic manner.The necessity and importance of the ANSA automated spot weld realization is shownexemplary in this paper by means of a component test: a seat console within the ECE R14test regulation.2 METALLURGICAL TESTS AND COUPON TENSILE TESTSBecause of the large number of different material and gage combinations in the entire vehiclebody, the spot weld characterization for FE analyses requires a vast number of experimentaltests. In addition to that, the presence of mainly three sheet joints rather than just two sheetjoints increases the number of combinations that need to be considered even more.In a first step physical tests of symmetrical two sheet joints (same gage and material) areperformed in order to understand the spot weld joint behavior itself. In a second step, tests ofnon-symmetrical two sheet joints (different gage and/or material) are of importance in orderto determine a so-called ‘rule of mixture’. The rule of mixture allows to calculate materialproperties and rupture parameters of non-symmetric joints, considering the weighting ofthickness, yield strength and peak load capacity. In a last step, the rule of mixture has to beproved and complemented by means of three sheet joint experimental testing.The detailed characterization of the spot weld is achieved by the determination of thematerial properties and diameter, as well as the identification of the rupture parameters. Twodifferent types of physical tests have to be performed in order to obtain such information: metallurgical tests (spot weld material properties and diameter)coupon tensile tests (rupture parameters)Both kinds of tests can be carried out for two and three sheet joints.2.1 Metallurgical testsThe metallurgical sample consists of two or three sheets joint welded with a few spot weldsand is used in order to study the microscopic properties of the joint. By carrying out suchmetallurgical tests the material properties and diameter of the spot weld can be determined.The proper preparation of metallographic specimens to obtain the spot weld microstructurerequires a rigid step-by-step process. In sequence, the steps include sectioning, mounting,
th5 ANSA & μETA International Conferencegrinding, polishing and etching. After this process, a microscopic examination follows: thehardness distribution of the joint can be extracted and three different subzones in the microstructure of the spot weld are identified: sheet basis material (BM), heat affected zone (HAZ)and nugget (Figure 1). These three different subzones are caused by the concentrated heatproceeding from the resistance spot welding process.Figure 1 – Hardness distribution and metallographic specimen (DP600)The hardness profile enables the calculation of the material properties of both HAZ andnugget (Figure 2). The yield curve of the basis sheet material is scaled by the hardness ratio:()()This information is taken into account and set as input in the FE model.Figure 2 – Hardness distribution and BM and nugget yield curve (CR3)The metallurgical cross-section also provides the information of nugget and HAZ diameter onthe basis of the determined dimension of each weld subzone. The diameter is measured inthree cutting angles 0 , 45 and 90 (Figure 3):Figure 3 – Cutting angles for diameter measurementIt has to be emphasized that the study of metallurgical samples with three sheet spot weldingcannot be overlooked. The metallographic specimen of a three sheet joint shows that the
th5 ANSA & μETA International ConferenceHAZ and nugget diameter between the top and the middle sheet can be completely differentthan between the middle and the bottom sheet (Figure 4). Such results have to beconsidered in the FE simulations and are shown in the next paragraphs.Figure 4 – Metallographic specimen (three sheets joint)2.2 Coupon tensile testsApart from metallurgical tests, different coupon tensile tests under various kinds of loadconditions have to be carried out in order to adequately determine the spot weld strengthvalues. Conventionally, tensile-shear, cross-tension and coach-peel specimens have beenperformed for the estimation of strength characteristic of spot weld joints (Figure 5). Howeverit is crucial to use extra specimen geometry with combined loading in order to be able toverify the FE simulation results. Coupon tensile tests are performed for both two sheet andthree sheet joints.Figure 5 – Tensile-shear, cross-tension and coach-peel specimenFrom the coupon tensile tests the force-displacement spot weld characteristic is extractedand the resultant peak force is captured. Figure 6 shows the force displacement curve for thethree above mentioned specimen geometries.
th5 ANSA & μETA International ConferenceFigure 6 – Tensile-shear, cross-tension and coach-peel test results (DP600, 1.00mm)Observing coupon test results, two remarkably different fracture modes of spot weld can bedistinguished: nugget interfacial rupture and nugget pullout rupture (Figure 7). The pulloutmode is generally the preferred fracture mode since the amount of energy absorbed by theweld is greater than in the case of interfacial fracture and has become the more commonrupture mode in full vehicle crashes. Nugget pullout fracture involves fracture in the peripheryof the weld within the HAZ or the base material rather than through the weld. Interfacialfracture is a less desirable fracture mode because a full separation of the joined sheetsoccurs as a result of a fracture through the weld, thereby providing less strength.Figure 7 – Nugget pullout and interfacial fracture mode3 FE MODELING3.1 Spot weld modelingIn FE crashworthiness analyses the sheet metal is normally represented with shell elementsand the spot welds with solid elements (Figure 8). Typically, a tied contact is used to
th5 ANSA & μETA International Conferenceestablish the connection among independent meshes; giving the solid weld elements athickness that is half the sum of the thicknesses of the welded shell elements. This results inan artificial thickness of the spot weld model, which may lead to exaggerated bendingmoments. However, the bending moment can be corrected in the FE solver, so that during itscalculation the true thickness of the spot weld is considered.Figure 8 – Spot weld in the reality and in the simulationThe discretization of the spot weld (Figure 9) can be modeled by a simple solid element(hexahedron) or by any number of solid elements, building a spot weld assembly (cluster).The cluster modeling has some advantages compared to the single element weld becausethe tied contact is more robust, more mesh independent, and at the same time the usage ofmultiple solid elements allows a better representation of the complex mechanism of spotweld rupture.Figure 9 – Modeling of the spot weld (single hexahedron, cluster4, cluster8 and cluster16)With a mesh size of the sheet panels between 3 and 4 mm, a weld discretization of eightsolid elements is fine enough to represent the weld while not adding additional mass to thespot weld. The element length of the weld assembly should not be larger than the elementlength of the sheet panel. In this paper the 8 cluster representation is chosen in order tomodel the spot weld nugget.It is important to consider the three different spot weld zones described in paragraph 2.1 inthe FE model. Therefore, the nugget and the HAZ need to be modeled. There are manypossibilities to represent the HAZ depending on the discretization of the nugget. In this paperthe modeling of the HAZ comprises one homocentric annulus around the nugget with theHAZ diameter. On the basis of the hardness distribution the diameter of the HAZ can beapproximated by scaling the spot weld diameter with a factor between 1.2 and 1.5 dependingon the thickness combination of the joint. In order to avoid an element length too small for theshells, the HAZ is modeled by eight shell elements (elements alignment equal to the weldcluster), where the inner element ring is half of the outer element ring (Figure 10).
th5 ANSA & μETA International ConferenceFigure 10 – Modelling of spot weld nugget and HAZ3.2 Material modelingIn order to choose the appropriate material modeling approach to characterize the spot weldseparation, the fracture mode of the spot weld has to be considered as well. Otherwise anincorrect modeling of the rupture mechanism may lead to over or under estimating the spotweld force.In this case, the interfacial rupture of the nugget is modeled with a simple material modelincluding a force or stress based rupture criteria. This approach is still the most commonlyused. It simplifies the spot weld separation, which is equivalent to the rupture of the spotweld solid elements.In contrast to the interfacial rupture, the much more complex tearing mechanism of thenugget pullout rupture requires a more comprehensive approach. In this case it is moresuitable to model the rupture in or adjacent to the HAZ using more sophisticated materialmodels. In this paper, a combined approach capturing both fracture modes is described inthe next paragraphs.
th5 ANSA & μETA International Conference4 ANSA SCRIPTING FUNCTIONALITY4.1 General RequirementsAs described in paragraph 3, different methodologies to discretize spot weld and spot weldrupture are available. Depending on the selected method, different spot weld rupture criteriaare valid. These criteria are derived from validated coupon test and saved in the spot weldtool box. The kind of spot weld and HAZ realization as well as the material and ruptureproperty parameter definition needs to be selected by the user within ANSA and the spotweld tool box.For this selection process, the following information from the simulation model is required foreach spot weld joint: Spot weld partner and stack up (Top/ Middle/ Bottom)Sheet thicknessBase material properties (MID/ SIGY/ ETAN).Within the spot weld tool box these data were used to calculate required material values anddefine spot weld joint specific values required for material rupture definition depending onrupture criteria: nugget material with rupture criteria (MID/ SIGY/ ETAN)HAZ material properties with rupture criteria (MID/ SIGY/ ETAN).To enable the interaction between ANSA and the spot weld tool box a specific numberingscheme had to be implemented.4.2 Implementation of Scripting FunctionalityIn order to achieve the FE-Representation as described in paragraph 3, a combination of thefollowing pre-processing actions has to be performed: The realization of the spot welds using the SOLID-NUGGET representation, offeredfor spot welds by the ANSA Connection Manager (see reference 1). During this step,the hexa-elements are generated as well as the shell mesh on the flange is modifiedaccordingly. The parallel call of user script, that performs additional actions on the FE-result, in apost-execution manner and automatically driven by the Connection Manager. TheANSA feature used is called Post realization script function. This is where the hexaelements and the HAZ-shells are modified accordingly: Dimensions, Nomenclature,Numbering rules as well as the generation of additional solver keywordsFor the purpose of this paper, focus will be given in describing the user script call as well asthe basic steps of the algorithm that are followed:a) First step is to identify the parts involved in the spot weld connection, andcategorize them according to their Flange sequence (top-middle-bottom sheet).b) According to the Flange sequence, the equivalent thickness is calculated as wellas the nominal diameter of the nugget.c) Given these values, the diameter per flange pair is determined for the
th5 ANSA & μETA International Conferencehexa-nugget as well as for the shell elements representing the HAZ.d) The dimensions of the hexas and the HAZ-shells are adjusted (their verygeometry).e) Properties and other solver keywords of the involved FE-elements areadjusted following rules: Nomenclature, Id-numbering directives andkeyword field values (such as the actual connector properties).4.3 Nomenclature and numbering rulesFirst the model assembly information regarding all connections within the model has to becaptured. Depending on model information whether an ANSA databank with defined genericconnector entity is available or a LS/DYNA input with respective contact definitions theprocedure is more extensive.To reduce the model size and to improve the model handling, same weld joints have to getthe same property and material IDs. Therefore, the joint information needs to be saved forfurther processing. A nomenclature rule for the solid nuggets and HAZ shells was introduced(see Figure 11Fehler! Verweisquelle konnte nicht gefunden werden.).To distinguish between solid nuggets and HAZ shells a type identifier is defined by using thefirst text block. The identifier for the spot weld nuggets includes the number of joint partners(SPW3 3 layered joint / SWP2 2 layered joint) and the stack up information (sheetA/B/C). This is followed by the corresponding weld diameter which will be calculated as afunction of sheet shell thicknesses. Last all partners of the weld joint are listed with the stackup position and corresponding shell property ID.For HAZ shells the name includes as well the Identifier and the considered stack up positionof the sheet. Then the zone diameter and the sheet thickness are listed followed again by allweld partners.Figure 11 – Nomenclature rule for spot weld joints within ANSA
th5 ANSA & μETA International ConferenceAn equivalent numbering rule for the corresponding property and material IDs for solidnuggets and HAZ shells was specified which enables an improved model handling.Using these rules ANSA is able to realize all welds by setting the connection post realizationfunction to the new Post realization script function. For realization it is possible to selectbetween different methods as described in paragraph 3. This enables the user to select therealization method corresponding to the coupon test validation models.Property ID and name as well as material ID for solid nugget and HAZ shells according thedefined rules will be then used as input parameters for the spot weld tool box. Within thetoolbox the determination of all material relevant data as stiffness and rupture criteria occurs.Depending on selected method the required material cards will be written.5 EXAMPLEThe following example is used to demonstrate how the application of more advanced weldmodeling techniques with new ANSA functionality combined with validated material data canimprove the prediction of structure performance.5.1 Regulation and Test Rig Description for Belt pullFulfilling ECE-R14 regulations for seat belt anchorages is mandatory for vehicle typeapproval. This standard was established to ensure the integrity of the seat belt anchoragesystems for effective occupant restraint.Two types of test devices (pelvic and torso block) are placed onto the seat and belted withoutbelt pretentioner and load limiter. The pull force is applied in a direction corresponding to theseating position at an angle of 10 5 above the horizontal plane of the vehicle. The seatbelt anchorages and seat anchorage must withstand the specific loads for a prescribed holdtime. The performance of the system is considered satisfactory if the system can sustain themaximum load. If this requirement is fulfilled permanent deformation or damage within theanchorage itself and the surrounding parts is permitted.In order to study the impact of different spot weld modeling techniques on the structuralperformance, a principal test setup using a typical design of a vehicle center floor and seatattachments was established that mimics the ECE-R14 type of loading. Due to the simplifiedtest set-up the behavior of individual spot welds could be monitored much easier leading to abetter database for the latter model correlation.5.2 Model validationThe most common modeling approach for spot welds is the discretization of the spot weld bya single solid element (Figure 12) without considering spot weld and material fracture.
th5 ANSA & μETA International ConferenceFigure 12 – Simulation model with simple spot weld definition (single hexahedron)Using this method, the structural integrity assessment is done by evaluating the plasticstrains and forces for each spot weld that might transfer high loads and comparing those to aset of limits (derived from principal spot weld tests) that might not be exceeded. Thesimulation result for the inner seat console of the passenger seat (here plastic strains) isshown in Figure 13.Figure 13 – Simulation results with simple spot weld definition (single hexahedron)@ simulation end timeIt is obvious that the weld connecting seat console and floor panel is highly loaded but thereis no indication for an integrity loss. The expectation was that only one weld may rupture.In contrast to this, the physical test using the principal setup showed a rupture of a largernumber of spot welds mainly driven by nugget pullouts (Figure 14).
th5 ANSA & μETA International ConferenceFigure 14 – Hard ware test correspond to simulation modelIn order to validate the simulation model, different spot weld modeling methods withrespective rupture criteria derived from coupon test validation were investigated.From the example above, it becomes clear that spot weld modeling with one hexa elementdoes not lead to satisfactory simulation results. Therefore, more advanced spot weldmodeling as described in paragraph 3.1 is required. Figure 15, Figure 16 and Figure showsimulation results for different spot weld modeling techniques. In all three cases, the weldnugget representation was changed from single hexahedron to a cluster consisting of 8 solidelements. But the models are different in the level of detail in geometrical spot weldrepresentation.In Fig. 16, all spot welds in the model have the same constant diameter of 5mm. Thismodeling technique is an improvement compared to the one hexa element modelingbecause it allows a better resolution of the stress state within the weld. But since all spotwelds in the simulation model are modeled with the same diameter, this approach can’tdistinguish between “stiff” and “weak” spot weld connections.This issue has been resolved by modeling weld nuggets with individual diameters dependingon corresponding sheet joint gauges (Figure 16). This geometrical representation of the weldnugget is more realistic, because it is commonly known, that standard procedures of sheetmetal welding lead to different weld nugget sizes. But still, this modeling approach is asimplification and therefore not generally sufficient to simulate spot weld behavior duringvehicle crash impacts, especially when spot weld rupture due to weld nugget pullout has tobe taken into consideration.In this case, a better representation of the weld joint would be to include the HAZ into thespot weld model. The result of this modeling is shown in Figure 17, where the simulationresults match the hardware test behavior very well in terms of rupture mode and carriedloads. Significant improvement of structure performance assessment can be achieved whenweld rupture is taken into consideration. Of course, in addition to HAZ meshing anappropriate material model for the HAZ has to be chosen to be able to model spot weldrupture due to nugget pullout. How to model material behavior including rupture of HAZ is noteasy and still topic of research projects (FOSTA P806/A262: Characterization and simplifiedmodeling of the fracture behavior of spot welds from ultra-high strength steels for crashsimulation with consideration of the effects of the joints on component behavior” / Ref. 2).
th5 ANSA & μETA International ConferenceFigure 15 – Simulation results of weld cluster8 with constant nugget diameter (5mm)@ simulation end timeFigure 16 – Simulation results of weld cluster8 with thickness dependent nugget diameter@ simulation end timeFigure 17 – Simulation results of weld cluster8 with thickness dependent nugget diameter and HAZ@ simulation time at maximum carried load within hard ware testBased on simulation models that are able to predict weld behavior correctly, designoptimization can be performed by CAE methods. Figure 18 shows the results of this designimprovement process in the simulation model and hardware test using weld cluster8 withthickness dependent nugget diameter and HAZ.These simulation results show clearly that a more advanced spot weld representation isbetter suited to model complex spot weld behavior in vehicle crash events. With the newlydeveloped ANSA Scripting functionality, it is possible to use these modeling techniques in anautomated and efficient way for larger scale models.
th5 ANSA & μETA International ConferenceFigure 18 – Simulation results optimized design of weld cluster8 with thickness dependent nugget diameter and HAZ @simulation end time / hard ware test6 SUMMARY AND OUTLOOKIt was shown that structure integrity assessment may be improved when using more detailedspot weld modeling techniques. For that purpose, it is necessary to adjust the spot welddiameter and the respective HAZ diameter for each spot weld joint depending on sheetgauge and stack up. Further, it is essential to apply material properties for the weld nuggetand the HAZ shells that are well suited to model spot weld behavior. From metallurgical andmechanical coupon testing, material weld stiffness properties can be derived in astraightforward manner and are managed within the spot weld toolbox. To derive appropriateparameters for weld joint rupture as additional content of the spot weld toolbox, is morecomplicated and still under development.The implementation of the new weld simulation method with the consideration of weldpartner dependent nugget diameter and HAZ into the vehicle development process requiresbesides the spot weld tool box a powerful pre-processor application to set up the simulationmodel fully automated. The new spot weld scripting functionality within ANSA offers thepossibility of weld rupture definition over a large number of spot weld joints within a fullvehicle simulation.7 REFERENCES(1)(2)ANSA version v14.1.0 User’s Guide, BETA CAE Systems S.A.,FOSTA P806/A262 “Characterization and simplified modeling of the fracture behaviorof spot welds from ultra-high strength steels for crash simulation with consideration ofthe effects of the joints on component behavior”
Figure 8 - Spot weld in the reality and in the simulation The discretization of the spot weld (Figure 9) can be modeled by a simple solid element (hexahedron) or by any number of solid elements, building a spot weld assembly (cluster). The cluster modeling has some advantages compared to the single element weld because
-Recognition of weld spot position -Automatic removal of weld spatter -Automatic ultrasonic weld spot inspection Ultrasonic spot welding quality inspection March 7, 2022 Slide 10 * Depending on spot weld quality and according to lab tests One combined tool -fully integrated solution Specification -Typical cycle time to inspect*: 3-6 sec
The optional chain can be adapted to the WELD CRAWLER (ZGN-SCN-WELD CRAWL ER). The chain can be used on a pipe that is up to 48" in diameter. 10048079 ZGN-SCN-WELD CRAWLER- OPT-AXIAL The axial weld solution option allows axial weld inspection using the WELD CR AWLER base (ZGN-SCN-WELD CRAWL
immunity (WFI) resist electromagnetic fields emitted by weld guns up to 100 kA/m. Weld-spatter Electromagnetic Weld Fields PROBLEM Hot welding-spatter (a.k.a. weld debris, weld slag, weld berries) sticks to sensor faces and bodies and causes pre
Weld toe radius is the radius which joining the fillet weld and base metal. Figure 2: Weld toe of cruciform shape fillet weld joint The objectives of this paper are as follows: Analysis and compare the effect of weld toe radius on tensile strength of fillet weld
Part of original welding kit. 6) T20004 Fillet Weld Tip ¼" fillet weld tip used for butt and right angle welds. 7) T20005 Fillet Weld Tips 3/8" fillet weld tip used for butt and right angle welds. 8) T20011 Fillet Weld Tips ½" fillet weld tip used for butt and right angle welds. 9) T20009 Ribbon Weld Tip used for welding thermoplastic
the quality of the weld bead generated by the welding system [10]. Distance sensors are also used to identify and track the weld seam during inspection [11]. Seam tracking systems have been developed to guide a weld gun along a weld seam to improve weld bead quality. For these systems, online inspection of the weld bead is still needed to
Busbar Weld If we consider the weld to the busbar to be a resistor, measuring the impedance is as simple as connecting a current source to the weld and measuring the voltage as shown in Figure 2. HI Current Source LO HI Voltmeter LO Weld Figure 2: Simple weld resistance circuit. The impedance of the weld is then calculated using Ohm's Law.
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